Don't use String8::empty

With the current definition, it's an alias of default constructor. This
has very different meaning from std::string empty, especially that
static method String8::empty can still be called from instance context.

One of such bugs are present in in ConversionHelperAidl.

Bug: 295394788
Test: make checkbuild
Change-Id: Ic466986bdf850a44c30c919450fd8b92ac68758d
diff --git a/media/libaudioclient/AudioTrack.cpp b/media/libaudioclient/AudioTrack.cpp
index 0fa814c..20b47b4 100644
--- a/media/libaudioclient/AudioTrack.cpp
+++ b/media/libaudioclient/AudioTrack.cpp
@@ -3448,7 +3448,7 @@
     if (output != AUDIO_IO_HANDLE_NONE) {
         return AudioSystem::getParameters(output, keys);
     } else {
-        return String8::empty();
+        return String8();
     }
 }
 
diff --git a/media/libaudiohal/impl/ConversionHelperAidl.cpp b/media/libaudiohal/impl/ConversionHelperAidl.cpp
index 7197bf2..46abfda 100644
--- a/media/libaudiohal/impl/ConversionHelperAidl.cpp
+++ b/media/libaudiohal/impl/ConversionHelperAidl.cpp
@@ -69,7 +69,7 @@
         // Re-parse the vendor-provided string to ensure that it is correct.
         AudioParameter reparse(String8(vendorParameters.c_str()));
         if (reparse.size() != 0) {
-            if (!values->empty()) {
+            if (values->length() > 0) {
                 values->append(";");
             }
             values->append(reparse.toString().c_str());
diff --git a/media/libdatasource/MediaHTTP.cpp b/media/libdatasource/MediaHTTP.cpp
index 58c1ce8..b1f2140 100644
--- a/media/libdatasource/MediaHTTP.cpp
+++ b/media/libdatasource/MediaHTTP.cpp
@@ -156,7 +156,7 @@
 
 String8 MediaHTTP::getUri() {
     if (mInitCheck != OK) {
-        return String8::empty();
+        return String8();
     }
 
     String8 uri;
diff --git a/media/libmedia/IMediaExtractor.cpp b/media/libmedia/IMediaExtractor.cpp
index eb436d1..ea50ee3 100644
--- a/media/libmedia/IMediaExtractor.cpp
+++ b/media/libmedia/IMediaExtractor.cpp
@@ -339,7 +339,7 @@
             if (source != NULL) {
                 instance.trackDescriptions.push_front(source->getFormat()->toString());
             } else {
-                instance.trackDescriptions.push_front(String8::empty());
+                instance.trackDescriptions.push_front(String8());
             }
             break;
         }
diff --git a/media/libmediaplayerservice/MediaPlayerService.cpp b/media/libmediaplayerservice/MediaPlayerService.cpp
index c08da0f..87c7f5e 100644
--- a/media/libmediaplayerservice/MediaPlayerService.cpp
+++ b/media/libmediaplayerservice/MediaPlayerService.cpp
@@ -2007,7 +2007,7 @@
 String8  MediaPlayerService::AudioOutput::getParameters(const String8& keys)
 {
     Mutex::Autolock lock(mLock);
-    if (mTrack == 0) return String8::empty();
+    if (mTrack == 0) return String8();
     return mTrack->getParameters(keys);
 }
 
diff --git a/media/libmediaplayerservice/include/MediaPlayerInterface.h b/media/libmediaplayerservice/include/MediaPlayerInterface.h
index 1cbd8a0..994c64a 100644
--- a/media/libmediaplayerservice/include/MediaPlayerInterface.h
+++ b/media/libmediaplayerservice/include/MediaPlayerInterface.h
@@ -147,7 +147,7 @@
         virtual bool        needsTrailingPadding() { return true; }
 
         virtual status_t    setParameters(const String8& /* keyValuePairs */) { return NO_ERROR; }
-        virtual String8     getParameters(const String8& /* keys */) { return String8::empty(); }
+        virtual String8     getParameters(const String8& /* keys */) { return String8(); }
 
         virtual media::VolumeShaper::Status applyVolumeShaper(
                                     const sp<media::VolumeShaper::Configuration>& configuration,